The present invention relates to power supplies and drive circuits, and more particularly, to a power supply for providing sinusoidal drive for a thin-film electroluminescent display.
Recent developments in thin-film electroluminescent displays (TFEL) have made such devices attractive for use in a variety of display systems. Such devices are capable of providing excellent display clarity in adverse ambient light conditions and are capable of extended operation over wide temperature ranges. Such features are highly desirable in many avionics systems and particularly in military aircraft which must be capable of operating in severe environments. Use of these devices has been restricted, however, due to the complexity and the power inefficiency of the drive circuits needed to power the TFEL displays.
In conventional TFEL power supplies and drive systems, square waves have been extensively used. While this technique is successful in providing the requisite operation of the TFEL displays, the operation is achieved only with high power dissipation and therefore low system efficiency. This is caused by the highly capacitive electrical nature of the TFEL devices which results in large reactive currents when driven by the square wave thereby resulting in the noted power dissipation. As a result of this operation, even though the luminous efficiency of the TFEL devices is high, power losses produced by the power supply and circuit conductors render the system much less efficient and therefore impractical in many environments.
In other instances, attempts have been made to drive the TFEL devices and accompanying displays by use of a resonating technique. In such instances, an inductance is used to produce a resonant circuit (using the inherent TFEL capacitance) which requires less power for system drive and results in less power dissipation. Such attempts, however, require highly complex circuits with additional losses and have not resulted in TFEL power supply and drive systems with much practical application. As will be understood, the above described technique requires separate circuits for each TFEL element to take advantage of the inherent capacitive reactance to produce the resonant circuit. Naturally, in the environments in which their use is intended, simplicity, low cost, and highly reliable operation with little power dissipation is required.
In order to overcome the above deficiencies, it has been suggested that sinusoidal drives be used in lieu of the square wave drive. As would immediately be apparent, a straight application of a sinusoidal drive in place of the square wave drive would not produce much additional benefit if the sinusoidal waveform were to be produced merely by amplifying a low level sinusoid. This would be due to the additional power loss incurred in the linear amplification of the low level sine wave form needed to convert a sine wave to a level sufficient to drive the TFEL devices. Again, such a system would not be desirable where highly power efficient circuits are required to drive the TFEL displays.
Accordingly, there is still a need for the development of power supplies capable of providing sinusoidal drive of greater power efficiency which may be used in connection with the supply of sinusoidal waveforms to TFEL devices and similar equipment. The present invention has therefore been developed to overcome the shortcomings of the above known and similar techniques and to produce an improved and highly efficient TFEL power supply.